Four out of six South African rhinos that were transferred to a park in southeast Chad in a bid to revive the endangered species have died, but not from poaching, conservationists say.

"An additional two black rhino carcasses have been discovered in Zakouma National Park in Chad, bringing the total mortalities to four, of the six that were reintroduced in May this year," the conservation group African Parks said in a press release.

"We can confirm that none of these rhinos were poached."

Six rhinos were introduced to Zakouma National Park under a joint initiative combining Chad, South Africa and the NGO, which successfully operates a number of wildlife parks in Africa.

They were put on a 4 800km flight from South Africa, accompanied by a team of vets.

Two were found dead last month.

Post-mortems and tests on blood, tissue and faeces have been sent to a lab in South Africa, African Parks said in the press release, which was issued on Friday.

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So far, there is no evidence of infectious disease or plant toxicity as a cause of death, it said.

Blood samples showed exposure to trypanosomes - a parasite transmitted by tsetse flies - but this is not suspected to be the cause of the tragedy, it said.

"Low fat reserves suggest that maladaptation by the rhinos to their new environment is the likely underlying cause, although tests to be undertaken on brain and spinal fluid may shed additional light on the exact cause of deaths," it added.

There are fewer than 25 000 rhinos left in the wild in Africa due to a surge in poaching, and only 5,000 of them are black rhinos.

The species is rated as critically endangered by the International Union for Conservation of Nature (IUCN).

Northern white rhinos disappeared from Chad several decades ago and the last western black rhino was recorded there in 1972, after decades of poaching pushed both subspecies to local extinction.

In July, outrage and a bitter row over responsibility flared when 11 black rhinos in Kenya died after being transferred to a new sanctuary, mainly due to toxic levels of salt in borehole drinking water.